I accidentally deleted a vertex
.. Loads of ways to do it.. but one option..

Assuming your mug is circular, Z-up..

Set Pivot to '3D Cursor' and Snap to 'Active', 'Vertex', affecting rotation




Select a good circle in your mug, which will yield its XY center
ShiftS > 'Cursor To Selected'





Select a region of faces you can copy to patch the hole
In Vertex Mode, select a suitable vertex to snap to, and make it active





ShiftDRZ duplicate and rotate the patch, and snap it into place.




After AltM merge > 'By Proximity'.. repair done.